Electrostatic, stylus driven digitizing panels are commonly configured with a flat panel display such that the display scan signal is coupled to the digitizing panels channel inputs. This causes the signal emitted from the display surface to capacitively couple with the digitizing panel's sensing layer. This in turn has the highly undesirable effect of introducing uncertainty in stylus position determinations so as to reduce the effective resolution of the display and digitizer system.
Flat panel displays typically have a scan signal with a square waveform of between 15 kHz and 20 kHz. The frequency of a typical pen or stylus is between 100 kHz and 200 kHz. Because the typical display horizontal scan signal is wide band, some harmonics of the signal fall within the normal pen or stylus frequency range. Since designers rarely have the opportunity to select the pen or stylus and display scan frequencies this interference is difficult to design around. Even where the designer has full control over pen/stylus and display frequencies the pen frequency often has an associated tolerance (e.g., where the pen frequency is not crystal controlled) which causes frequency uncertainties. This problem is compounded where different display resolutions (and frequencies) are utilized.
It is, therefore, a primary object of the present invention to provide a method and an apparatus for reducing pen or stylus and display signal interactions. Another object of the present invention is to provide a method and apparatus for dynamically adjusting the display horizontal scan frequency so as to reduce pen or stylus and display signal interactions. A further object of the present invention is to provide a method and apparatus for allowing a user to optimize the display horizontal scan frequency so as to reduce pen or stylus and display scan signal interference. Yet a further object of the present invention is to provide a method and apparatus for automatically adjusting the horizontal scan frequency of a display and digitizer system whenever signal interference is detected.